Video card failed some of Hardware Diagnostic tets

Hi can any one help me please regarding my display problems. Last week suddenly my desktop display has gone. Then I did some Hardware Diagnostics using HP software. It failed on couple tests . I think one related to heat sink and other related to some stress test. Not rememberd exact technical wording. At first instance I reloded OS. Then display worked some time. But after some time it's gone again. If i turn off CPU and restart again it will work some time. But the problem comes back.

Can any once suggest me If I replace my cpu video card will the problem can be rectified. My video card details are

PCI Express x16 graphics card - NVIDIA GeForce 8400GS (Ranger), 256Mb, 64-bit memory interface. Searched for this product . Did not find same product. Can any one suggest what type of card will be suitable for my CPU. My CPU model number is HP Pavilion a6235.uk. At present my video card is 256 mb, if i want can i increase the memory size. My existing RAM CPU is 3GB.

Unplug the PC and open it up. Clean out all the dust. Carefully remove and replace all the cables going to the motherboard one at a time. Do the same for the memory dimms and the video and sound cards if you have any. You might want to buy a can of compressed air to blow the dust out of the CPU heat sink. Plug your PC back in and give it a go.

The NVIDIA 8400 GS has a SPDIF connector so consider buying a card to support that feature. This NIVIDA GT 240 has the SPDIF connector.